1. Field of the Invention
This invention relates to systems and methods for mooring deep water, mineral production, tension leg platform (TLP) and tender vessels.
2. Related Art
Recently, relatively smaller platforms have been developed for deep sea operations where marginal production does not merit the use of a full scale tension leg platform (TLP). These marginal platforms use tension leg mooring, like conventional tension-leg platforms, but comprise smaller floatation structures. Tension-leg mooring typically comprises rigid, single-piece tendons for anchoring the structure to the sea floor, like that disclosed in Monopod TLP Improves Deepwater Economics, PETROLEUM ENGINEER INTERNATIONAL (January 1993), incorporated herein by reference.
It is desirable to use the TLP as a production platform, however, the weight and cost of the TLP platform increase significantly with increasing water depth and payload. Monohull vessels provide greater capacities, but they can hardly support the great weight of the risers which transport minerals from wells on the sea floor when disconnect from the operating site is required. Therefore, it is desirable to use a small riser wellhead TLP platform to suspend the risers and support the wellheads and a disconnectable tender vessel to receive, process, and export the produced minerals. The well streams are chocked and manifolded on the TLP riser platform.
Close mooring of the TLP riser platform and the production tender vessel allows for light weight flexible hoses to be used to transport the minerals. However, as the two vessels are brought into close proximity so that minerals can be transported between the TLP and the tender vessel, environmental loads induce excessive displacements on the two vessels and large loads on the transfer system.
Most mooring systems are heavy for larger water depths and rough environments so that they require larger production and tender vessels. In marginal deep sea production, larger vessels are not economical. Previously, mooring systems have been provided for single vessels. For example, U.S. Pat. No. 5,045,415, issued to Marshall on Oct. 8, 1991, incorporated herein by reference, discloses a mooring bridle. The reference suggests that different vessels may be moored within the same mooring bridle at any one time, but does not suggest connecting multiple ships to the same mooring bridle at the same time. In order to moor vessels relative to each other so that minerals may be transferred by light weight, flexible systems, the prior systems require a significant number of mooring lines increasing the overall weight of the system. Therefore, there is a need for a mooring system of two vessels which stabilizes the two vessels relative to each other without excessive loading of the vessels by the mooring system.
Another problem associated with the prior mooring systems is associated with the installation of the mooring bridle. In general, the mooring bridle includes anchors attached to the sea floor, anchor lines having one end attached to the anchor and the opposite end attached to a floating anchor buoy, and bridle lines extending between the anchor buoys. The anchors are spaced about a central mooring location so that attachment of the vessel to the mooring bridle with vessel lines holds the vessel at or near the central mooring position. However, when installed, the anchors are typically positioned at substantial depths. Attachment of the anchor line to the anchor is generally made using a remotely operated vehicle (ROV). The ROV pulls one end of the anchor line down to the installed anchor and aligns rings attached to the anchor with rings attached to the end of the anchor line. Once aligned, a dowel is inserted through the aligned rings to attach the anchor line to the anchor. In practice, the process of attaching the anchor lines to the anchors is very difficult, time consuming, and costly.
This problem is exacerbated because the anchor lines must be replaced periodically. Therefore, the difficult process of attaching the anchor lines to the anchors must be repeated, often more than once, during the lift of the mooring bridle. Accordingly, a more convenient method for attaching the anchor lines to the anchors is desired.